1. Field of the Invention
The present invention relates to pre-treatment process prior to a thin film deposition. More particularly, the present invention relates to method for removing native oxide.
2. Description of the Related Art
In the fabrication of integrated circuits, surface purity has a critical influence on the quality of subsequent processing steps (such as forming a gate oxide layer, depositing HSG, or a metal layer). Thus, it is necessary to perform a pre-treatment process before subsequent processing steps, in order to remove organic and inorganic materials, subatomic particles and native oxide attached to the silicon surface. Most pretreatment steps include acid wash, alkaline wash, chemical oxidation and diluted hydrofluoric acid treatments.
Native oxide refers to the silicon oxide (SiO2) layer that forms on the surface of a wafer resulted from the oxidation of silicon atoms when the wafer has been exposed to an oxide-containing environment such as oxygen or water. Since the silicon oxide layer is formed naturally with water vapor or oxygen present in the air, it is called native oxide. This native oxide layer has a harmful effect on the quality of the thin film (HSG, metal silicide) formed in a subsequent step. Thus, in most semiconductor fabrication processes, a pre-treatment step is carried out to remove native oxide from the surface of the silicon wafer before forming the thin film. In this way a better quality thin film is attained in the latter stages of the fabrication process.
Currently, there are not many good methods for removing native oxide. The methods described below are the most suitable ones in use at the present time. One method uses gaseous plasma such as Argon (Ar) plasma to etch away the native oxide layer on the surface of a silicon wafer. However, a plasma causes damage to the silicon surface. Another method uses hydrogen gas to replace the oxygen in the native oxide, restoring silicon oxide to silicon. Using hydrogen atoms does not cause as much damage to the silicon surface. However, the method using hydrogen atoms requires that the temperature be raised to more than 700 degree Celsius. At such a high temperature, amorphous silicon will be transformed into polysilicon. Thus, this method can not be used to remove native oxide on the surface of amorphous silicon. Additionally, high temperature also alters the shape of the P/N junction in the substrate, which has a harmful effect on the conductivity of the device. Moreover, after the native oxide layer has been removed, the silicon wafer must be removed from the vacuum of the reaction chamber. When the silicon wafer once again comes into contact with air, the surface will be oxidized by the oxygen components in the air, creating a second native oxide layer.
As embodied and broadly described herein, the invention provides a method for removing native oxide, wherein the method uses hydrogen to react with the native oxide layer, displacing the oxygen atoms in silicon dioxide and thus achieving the goal of native oxide removal. Additionally, ultra-violet (UV) laser irradiation enhancement is used to reduce the reaction temperature to approximately between about 200xc2x0 and about 700xc2x0 C. Thus, the goal of low-temperature native oxide removal can be attained.
The invention provides a thin film deposition pre-treatment step used to remove native oxide on the surface of a substrate, comprising the steps below. A semiconductor substrate is placed in a reaction chamber, wherein the pressure in the chamber is approximately between 10xe2x88x926-10xe2x88x922 torr. The substrate is heated to between 200xc2x0 and 700xc2x0 C. Hydrogen is passed through the reaction chamber and the substrate is irradiated with an UV laser. After the aforementioned treatment process has been conducted, the substrate can be transported to another reactor without exposing to air where thin film deposition is performed. As a consequence, a second native oxide is prevented from forming which enables the goal of a preferred film to be attained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.